Cathode ray tube



Nov. 15, 1966 R. E. BENWAY CATHODE RAY TUBE Filed Nov. 20. 1961 Z2 32 24 Z6 Z8 IN VEN TOR F055 5551/11 42 United States Patent 3,286,113 CATHODE RAY TUBE Robert E. Benway, Marion, Ind., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. 20, 1961, Ser. No. 153,451 8 Claims. (Cl. 31382) This invention relates to cathode ray tubes and particularly to an electron gun therefor having relatively high drive-sensitivity.

In many cathode ray tube applications, such as portable television receivers, it is desirable that the electron gun of the cathode ray picture tube have high (good) drivesensitivity, i.e., that a small change of signal voltage between the cathode and the control grid of the picture tube effect a relatively large change in beam current intensity. Conventional electron guns of the type including a control electrode having a transverse plate with a circular beamforming aperture therein do not exhibit the desired degree of beam control sensitivity.

Various schemes of the prior art have been proposed for the purpose of obtaining good-drive sensitivity. Among there are the provision of electron guns having control electrodes with apertures whose areas are effectively subdivided into smaller elemental areas. Examples of the subdivided electrode apertures of the prior art are those having inwardly projecting tangs and those with crossed wires or meshes overlying a circular beam forming aperture. The theory of these proposals is that a greater ratio of aperture boundary (electrode edge around aperture subdivisions) to total aperture area is provided, which in turn provides greater sensitivity of beam intensity control. While this theory is sound, most prior art approaches along these lines have been fraught with the problem of harmful beam breakup, which is due to the subdivision of the aperture area by the tangs, cross wires, meshes, or the like, and which is difficult, if not impossible, to satisfactorily focus to a desired single small spot on the phosphor screen. As a result, poor resolution is obtained.

In electron guns using control electrodes with subdivided apertures, attempts have been made to prevent beam breakup by spacing the control electrode extremely close to the cathode. This, however, presents a problem of fabrication due to the close spacing tolerances required.

It is therefore an object of this invention to provide a new and improved cathode 'ray tube having an electron gun of the subdivided-control-electrode-aperture type which has very high sensitivity and which at the same time is not fraught with problems of beam breakup or close spacing tolerances.

Briefly, according to features of this invention, a cathode ray tube includes an electron gun having a subdivided control electrode aperture which preferably includes a central circular aperture surrounded by an annular aperture concentric therewith. In operation of the tube, the electron streams passing through the different aperture subdivisions are individually converged to a common or coincident crossover at a point on the longitudinal axis of the gun. This cross over is imaged as a single small spot on the phosphor screen.

In the drawings:

FIG. 1 is a side elevation view partly in section and with parts each broken away of a cathode ray tube according to the invention;

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FIGS. 2 and 3 are enlarged longitudinal section and end plane views, respectively, of a portion of the tube of FIG. 1; and

FIG. 4 is a more greatly enlarged section of a portion of the tube of FIG. 1 illustrating the electron optics thereof.

Referring to FIG. 1, a cathode ray tube 10 includes an envelope comprising a neck section 12, a faceplate 14, and an interconnecting funnel section 16. An electron gun 18 in the neck 12 is adapted to project an electron beam onto a target in the form of a luminescent screen 19 on the faceplate 14. The screen 19 may be of any suitable type such as one comprising a layer 20 of phosphor material on the faceplate with a superimposed film 21 of evaporated aluminum thereupon.

The electron gun 18 comprises a plurality of coaxial, tubular, centrally-apertured electrodes including a control electrode cup 22, a screen electrode cup 24, and a focussing system including a first anode 26, a focus-sing ring 28, and a second anode 30. These electrodes are mounted in coaxial spaced relationship along a pair of insulator rods 32 by U-shaped mounting studs 34 which are fixed to the electrodes and embedded in the insulator rods. A tubular cathode 36 is mounted in a centrally apertured insulator disk 38 which is in turn fixed within the control electrode cup 22. The cathode 36 is closed at one end and coated with suitable materials to provide a planar electron emissive surface 40.

A conductive coating 42 is provided on the internal surface of the funnel 16 and is connected to the luminescent screen 19 and to the second anode 30 through a plurality of spring snubbers 44 attached to the second anode. A high voltage contact terminal indicated schematically by the arrow 46 is provided for applying a suitable voltage to the coating 42, the anodes 26 and 30, and the phosphor screen 19.

The neck 12 is closed at its free end with a stem structure 48 which includes a plurality of lead-in conductors 50 for applying suitable voltages to electrodes of the electron gun 18.

In FIGS. 2 and 3, the control electrode cup 22 is shown to have an aperture 52 in its planar end wall 54. A thin flat metal plate 56 is fixed to the end wall 54 inside the control elect-rode cup 22 and overlies the cup aperture 52. The plate 56 is provided with a circular aperture 58 coaxial with the cup aperture 52. A conductive ring 60 having a diameter smaller than the plate aperture 58 is concentrically mounted in the aperture 58 by a plurality of support rods 62. By virtue of the ring 60, the plate aperture 58 is effectively subdivided so as to form a small central circular aperture 64 and a surrounding, substantially complete annular aperture 66. Although the annular aperture 66 is in fact itself subdivided by virtue of the support rods 62 into two approximately half-annular areas, for the purpose of brevity and clarity, it will herein simply be referred to as an annular aperture.

The circular aperture 64 and the annular aperture 66 of the plate 56 serve as the beam forming and control aperture of the control electrode 22 and as such are disposed opposite the emissive surface 40 of the cathode.

The cup aperture 52 in the end wall 54 of the control electrode cup is of a diameter larger than the outside diameter of the annular aperture 66. As a result, that portion of the end wall 54 forming the circular boundary of the cup aperture 52 provides a shoulder 68 concenpurpose of shaping a beam-converging, electron-accelerating field as hereinafter described. trically surrounding the beam control aperture 58 for the In contrast with prior art electron guns having subdivided control electrode apertures, electron gun construction and operation according to this invention results in each individual electron stream which passes through each subdivision of the control electrode aperture being individually converged to a common or coincident crossover. Such a convergence permits imaging of a finite spot by the focussing field onto the luminescent screen, thus in turn affording good image resolution.

Referring to FIG. 4, in operation of the electron gun 18, electrons are emited from the emissive surface 40 of the cathode 36 and drawn through the control electrode beam-forming apertures 64 and 66 to provide a solid, central circular stream 70 and a surrounding annular stream 72. The electron streams 70 and 72 are individually converged to a common first crossover 74 at a point on the longitudinal axis of the gun by convergent electrostatic fields established in the regions between the control and screen electrodes 22 and 24. The crossover 74 is then imaged by focusing fields established by the focusing system including electrodes 26, 28, and 30 onto the luminescent screen 19. Such operation of the electron gun 18 is achieved by properly relating the electrode dimensions and spacings and applying suitable voltages to the electrodes as hereinafter more fully described.

Convergence of the circular electron stream 70 to the crossover 74 as referred to in the preceding paragraph involves formation of the circular stream into a somewhat conical shape between the cathode 36 and the cross over 74. The apex of the electron cone thereof occurs at and forms the crossover. Convergence of the annular electron stream 72 to the cross-over 74 as referred to in the preceding paragraph involves both a thinning down of the thickness of the annular Wall of the electron stream 72 and a convergence of this annular wall to the crossover point 74 of the circular stream 70.

From the crossover 74, the electrons of both the circular stream 70 and the annular stream 72 proceed toward the luminescent screen 19 in divergent paths until they encounter the focusing field of the electrodes 26, 28, and 30. This focusing field then reconverges the electron stream-s 70 and 72 to a point, or small spot, on the luminescent screen 19. This focusing action may he described as an imaging of the comm-on crossover 74 on the luminescent screen.

In FIG. 4 an ideal point-crossover 74 is illustrated for the purpose of more clearly explaining the operation of the invention. However, in practice, as is true with any electron stream crossover, the crossover 74 actually has a finite length along the gun axis and a finite diameter. As is well known in the art, realization of an ideal pointcro-ssover in evacuated devices is never fully obtained due to space charge mutual repulsion forces between the electrons of the stream.

The electron optics of the gun 18 produce the desired coincident crossover 74 of bot-h the circular and annular electron streams 70 and 72 in a way illustrated in FIG. 4. Suitable voltages are applied to the electrodes of the gun 18 to provide an accelerating electrostatic field between the control and screen electrodes 22 and 24. Two equipotential lines 76 and 78 of this accelerating field are shown for the purpose of more clearly explaining the operation of the gun 18.

As shown by the equipotential line 78, the accelerating field dips into the central aperture 64 to provide a relatively strong convergent field portion 8t) localized opposite the central circular aperture 64. The convergent field portion 80 acts to focus the circular electron stream 70 to the first crossover 74 as hereinbefore described. Similarly, the accelerating field dips into the annular aperture 66 to provide a relatively strong convergent field portion 82 localized opposite the annular aperture 66. The annular convergent field portion 82 serves to focus the annular electron stream 72 so that the thickness of the wall of the annular stream is thinned down as it proceeds away from the cathode emissive surface 40.

Were the accelerating field between the control and screen electrodes 22 and 24 planar, i.e., without concave curvature as is illustrated by the line 76, the effect of the convergent field portion 82 would be to simply focus or thin the wall of the annular electron stream 72 to an annular crossover with the diameter of the annular stream 72 remaining the same. If such were the case (which it is not), the wall of the annular stream 72 would not be converged to a point crossover but rather would proceed along a cylindrical path generally perpendicular to the cathode emissive surface 40. However, the accelerating field between the control and screen electrodes 22 and 24 is not planar but rather is concave toward the screen electrode 24 so that the annular convergent field portion 82 is tilted toward the central axis of the gun 18 to form the annular convergent field portion 82 into a somewhat frusto-conical shape with the large end of the frustum adjacent to the control electrode 22. The central convergent field portion 64 is substantially planar with respect to the central axis of the gun, in order to focus the central beam without deflecting it.

The shoulder 68 on the control electrode 22 is that portion of the electrode structure of the gun 18 which primarily produces the concavity of the acceleratin-gfield. The shoulder 68 causes the accelerating field as illustrated by the equipotential lines 76 and 78 to be spaced further away from the plane of the apertured control electrode plate 56 than is the central portion of the accelerating field, thus providing the desired concavity.

Other means, such as a concave shaping of the apertured end walls of the control and screen electrodes 22 and 24, could be used to produce the desired concavity of the accelerating field. The use of the shoulder 68 is preferred because of its simplicity and adaptability as to size selection for selection of the degree of concavity as hereinafter described.

In order to obtain optimum operational characteristics of the electron gun 18, the beam control apertures 64 and 66 in the control electrode 22 are of such relative size as to provide simultaneous cutoff of their respective electron streams 70 and 72. In the case of each of these apertures, as its size is decreased, cutoff of its electron stream is obtained with a smaller voltage difference between the cathode and control electrode. Cutoff of the two electron streams 70 and 72 can also be selected by selecting the size of the aperture 84 in the screen electrode 24. As screen electrode aperture 84 is increased in diameter, the voltage difference between the cathode and control electrode necessary to produce cutoff is increased. However, varying the size of the screen electrode aperture 84 varies the cutoff voltages of the circular and annular electron streams 70 and 72 at different rates. Thus, size selection of screen aperture 84 provides a way of contributing to equalization of cutoff for the two electron streams 70 and 72.

Varying the size of the beam control apertures 64 and 66 also affects the strengths of the convergent field portions represented by field lines 80 and 82, which determine the distance from the cathode surface 40 of the crossover 74. Decreasing the size of one of these apertures increases the strength of its convergent field portion 80 or 82, which in turn moves the crossover 74 of its electron stream 70 and 72 closer to the control electrode 22. Therefore, in designing the electron gun 18, the beam control apertures 64 and 66 and the screen electrode aperture 84 are made of a selected size to obtain both simultaneous cutofl of the circular and annular electron streams 70 and 72 and a crossover of these streams at the same distance from the cathode surface 40.

In order to obtain a concidence of the crossovers of both the circular and annular electron. streams 70 and 72, the concavity of the accelerating field is made to be of a selected magnitude. The greater the concavity of the accelerating field, i.e., the smaller its radius of curvature, the greater is its strength and the" closer the crossover 74 of the annular electron stream is to the control electrode 22. Either an increase of the thickness of the end plate 54 to provide greater height to the shoulder 68 or a decrease of diameter of thecup aperture 52 .to move the shoulder radially inward serves to increase the concavity, and thus the strength, of the accelerating field.

Adjustment of the degree of concavity of the accelerating field can also be obtained by varying the diameter of the aperture 84 in the screen electrode 24.

The apertured control electrode plate 56 is made to have a thickness which will most easily permit obtaining of the desired electron optics. As its thickness is increased, other parameters remaining fixed, electrode spacings are affected. This in turn affects the strength of the convergent field portions 80 and 82 and consequently the location of the crossovers of the electron streams 70 and 72. If the plate 56 is made too'thick, the cutoff characteristic i degraded beyond a desired limit.

The width of the ring 60 dividing the control electrode aperture into its circular and annular parts is preferably made as small as practical so as to intercept as few electrons as possible and thus maximize the amount of cathode emission for any given drive voltage.

The preferred mode of operation of the gun 18 is in grounded grid drive with the cathode biased at cutoff and driven with a negative signal from cutofi to zero bias. The gun 1-8 can, however, be operated in other modes.

One embodiment of the invention successfully operated in the preferred mode, comprises an electron gun 18 of the general type used in a commercial 23AHP4 type television picture tube, but in which the control and screen electrodes are specially fabricated as follows:

Mils Thickness of control electrode end wall 54 7 Thickness of control electrode apertured plate 56 1 Thickness of apertured end wall of screen electrode 24 10 Diameter of cup aperture 52 60 Diameter of circular aperture 64 10 Inside diameter of annular aperture 66 Outside diameter of annular aperture 66 40 Diameter of screen electrode aperture 84 31 Spacing between cathode 36 and control electrode plate 56 4 Spacing between control electrode wall 54 and screen electrode 24 4 Suitable D.C. voltages for operating such a gun in the preferred mode are as follows:

Cathode 36 volts +28 Control electrode 22 volts 0 Screen electrode 24 volts +50 Anodes 26 and 30 and luminescent screen 19 kilovolts +16 Focus ring 28 (as required to obtain proper focus) volts 0 to +400 With the above noted structural dimensions and operating voltages, the gun 18 has a cutoff of only 28 volts and produces a zero bias current of 2200a amps.

With slightly different structural dimensions in another embodiment of the invention a cutoff of, for example, only volts and a zero bias current capability of about 1350 amps may be obtained.

What is claimed is:

1. A cathode ray tube comprising a luminescent screen and an electron gun adapted to project an electron beam onto said screen, said gun comprising a plurality of electrodes aligned along a central axis, said electrodes including a cathode having a planar electron emitting portion, a control electrode including a planar portion having a subdivided beam control aperture of given diameter opposite said cathode portion, an electron accelerating electrode for accelerating electrons emitted from said cathode portion into separate streams passing through the subdivisions of said aperture, said accelerating electrode having a single aperture of given diameter spaced from and coaxial with said beam control aperture, means including a low potential field-forming electrode adjacent to said control electrode on the side thereof opposite from said cathode and having a single aperture of substantially greater diameter than said given diameters coaxial with said beam control aperture for creating between said control and accelerating electrodes a convergent electric field for individually converging and focusing each of said electron streams to a common crossover on said central axis, and means for converging and focusing said streams between said crossover and said screen to image said crossover on said screen.

2. A cathode ray tube as in claim 1, wherein, said control electrode has a central circular aperture and a substantially annular aperture concentrically substantially surrounding said circular aperture.

3. In a cathode ray tube an electron gun including in aligned disposition a cathode, and control, accelerating, and focusing electrodes, said control electrode having opposite said cathode a central circular aperture and an annular aperture concentrically substantially surrounding said circular aperture, and means including an annular shoulder on said control electrode on the side thereof opposite from said cathode concentrically surrounding and spaced radially from said annular aperture for establishing a circular convergent field opposite said circular aperture and an annular convergent field opposite said annular aperture, said apertures being so sized as to have the same cutoif voltage, said convergent fields being so shaped and of such intensity as to converge the electron streams through their respective apertures to a common crossover point on the axis of said gun.

4. A cathode ray tube com-prising a luminescent screen and an electron gun adapted to project an electron beam onto said screen, said gun including in aligned disposition a cathode, and control, accelerating, and focusing electrodes, said control electrode having opposite said cathode a central circular aperture and an annular aperture concentrically substantially surrounding said circular aperture, and means including an annular shoulder on said control electrode on the side thereof opposite from said cathode concentrically surrounding and spaced radially from said annular aperture for establishing an accelerating field between said control and accelerating electrodes opposite their apertures which is concave toward said accelerating electrode and which has a circular portion dipping into said circular aperture enforcing convergence of the electron stream passing therethrough to a point on the axis of said gun and an annular portion dipping into said annular aperture enforcing convergence of the electron stream passing therethrough, the strength of said annular convergent field portion and the curvature of concavity of said accelerating field being such as to converge said annular stream to said point, the relative dimensions of said circular and annular apertures being such that the beam cutoff voltage is substantially the same for each of said apertures.

5. A cathode ray tube as in claim 1, wherein said control electrode comprises a relatively thick wall portion having an opening larger than said aperture, and a relatively thin plate containing said aperture mounted over said opening on the side towards said cathode, whereby the wall of said opening constitutes said shoulder.

6. A cathode ray tube as in claim 1, wherein said field-forming electrode is in contact with said control electrode.

7. A cathode ray tube as in claim 1, wherein the diam- 7 8 eter of said accelerating electrode aperture is smaller than "2,919,380 12/ 1959 Barnett 313 -821 X the diameter of said beam control aperture. 2,880,342 3/1959 Frenkel 31382.1 X 8. A cathode ray tube as in claim 1, wherein said com- 2,971,118 2/ 1961. B di k -313 g2 X anon crossover is located between said accelerating elec- 2,975,315 3/1961 Szegho 313 251 X tro and i n. 5- 2,988,658 6/1961 Skellett 313-82 References Cited by the Examiner I HERMAN KARL S a H B ACH, Primary Examiner. UNITED STATES PATENTS 2,721,287 10/1955 VnOrmer 313-;-.s2"X" ARTHUR GAUssExamme" 2,862,144 11/1958 McNaney 1 31 3 g'2;1x s. CHATMON, JRQ'Assis raht Examiner. 

1. A CATHODE RAY TUBE COMPRISING A LUMINESCENT SCREEN AND AN ELECTRON GUN ADAPTED TO PROJECT AN ELECTRON BEAM ONTO SAID SCREEN, SAID GUN COMPRISING A PLURALITY OF ELECTRODES ALIGNED ALONG A CENTRAL AXIS, SAID ELECTRODES INCLUDING A CATHODE HAVING A PLANAR ELECTRON EMITTING PORTION, A CONTROL ELECTRODE INCLUDING A PLANAR PORTION HAVING A SUBDIVIDED BEAM CONTROL APERTURE OF GIVEN DIAMETER OPPOSITE SAID CATHODE PORTION, AND ELECTRON ACCELERATING ELECTRODE FOR ACCELERATING ELECTRONS EMITTED FROM SAID CATHODE PORTION INTO SEPARATE STREAMS PASSING THROUGH THE SUBDIVISIONS OF SAID APERTURE, SAID ACCELERATING ELECTRODE HAVING A SINGLE APERTURE OF GIVEN DIAMETER SPACED FROM AND COAXIAL WITH SAID BEAM CONTROL APERTURE, MEANS INCLUDING A LOW POTENTIAL FIELD-FORMING ELECTRODE ADJACENT TO SAID CONTROL ELECTRODE ON THE SIDE THEREOF OPPOSITE FROM SAID CATHODE AND HAVING A SINGLE APERTURE OF SUBSTANTIALLY GREATER DIAMETER THAN SAID GIVEN DIAMETERS COAXIAL WITH SAID BEAM CONTROL APERTURE FOR CREATING BETWEEN SAID CONTROL AND ACCELERATING ELECTRODES A CONVERGENT ELECTRIC FIELD FOR INDIVIDUALLY CONVERGING AND FOCUSING EACH OF SAID ELECTRON STREAMS TO A COMMON CROSSOVER ON SAID CENTRAL AXIS, AND MEANS FOR CONVERGING AND FOCUSING SAID STREAMS BETWEEN SAID CROSSOVER AND SAID SCREEN TO IMAGE SAID CROSSOVER ON SAID SCREEN. 